Field of the Invention
The present disclosure relates to an organic light emitting display device; and more particularly, the organic light emitting display device of the present disclosure has reduced crosstalk, and improved luminance and image quality, by reducing a coupling effect caused by a parasitic capacitor.
Discussion of the Related Art
An organic light emitting display device is a self-light emitting display device and does not need a separate light source unlike a liquid crystal display device, and, thus, the organic light emitting display device can be manufactured into a lightweight and thin form. Further, the organic light emitting display device is advantageous in terms of power consumption because it is driven with a low voltage. Also, the organic light emitting display device has a high response speed, a wide viewing angle, and a high contrast ratio (CR). Therefore, the organic light emitting display device has attracted attention as a next-generation display device.
The organic light emitting display device includes a plurality of lines and a plurality of sub-pixels connected thereto. Each sub-pixel includes an organic light emitting element and a pixel circuit including a thin film transistor electrically connected with the organic light emitting element and a storage capacitor.
In recent years, as resolution of an organic light emitting display device has been improved, a gap between lines, and a gap between lines and a pixel circuit have become smaller. Because the gap between lines and a pixel circuit has become smaller, the pixel circuit and the lines form a parasitic capacitor and a plurality of signals is coupled. In particular, a coupling effect in an organic light emitting display device of the prior art can be generated between a data line and a gate electrode of a driving thin film transistor. Due to the coupling effect, a driving current supplied to an organic light emitting element is changed. Thus, an image quality of the organic light emitting display device of the prior art may deteriorate.
To be specific, deterioration in image quality may cause problems such as crosstalk and deterioration in luminance. The crosstalk, for example, may refer to a phenomenon that when white and black patterns are displayed on a display device, and if there is a great difference in electrical load between areas of the display device, a coupling effect increases. Thus, the white pattern increases the luminance in the black pattern. The deterioration in luminance may refer to a phenomenon that when an image signal intended to be displayed is input to a pixel, a coupling effect increases, and, thus, the luminance of the pixel decreases. That is, the coupling effect may cause generation of an unwanted electric field which deteriorates an image quality.
FIG. 1A and FIG. 1B are a schematic plane view and a cross-sectional view, respectively, illustrating a coupling effect caused by a parasitic capacitance Cp in an organic light emitting display device 100 of the prior art. The organic light emitting display device 100 includes a substrate 110, a driving thin film transistor 120, a storage capacitor 130, a switching thin film transistor 140, a data line 151, a VDD line 152, a gate line 153, and an organic light emitting display device 160 (refer to FIG. 1A). The data line 151 and the gate line 153 are disposed so as to intersect each other on the substrate 110. The organic light emitting element 160 includes an anode 161, an organic light emission layer 162, and a cathode 163. The organic light emitting element 160 emits a light having a wavelength in a visible ray range. Further, the luminance of the light is determined on the basis of an amount of a driving current input through the anode 161. The amount of the driving current is regulated by the driving thin film transistor 120 connected with the anode 161.
The driving thin film transistor 120 includes an active layer 121, a gate electrode 122 overlapping the active layer 121, and an input electrode 123 and an output electrode 124 connected with the active layer 121. The active layer 121 may refer to a channel or a semiconductor layer.
The input electrode 123 of the driving thin film transistor 120 is connected with the VDD line 152. For example, the VDD line 152 may be connected with the input electrode 123 through a connection line 154 disposed under the VDD line 152. The driving current transferred by the VDD line 152 is input to the anode 161 through the active layer 121 and the output electrode 124. The amount of the driving current flowing in the active layer 121 of the driving thin film transistor 120 is regulated by a voltage of an image signal supplied through the data line 151.
An overlap area between a first electrode 132 and a second electrode 131 may be considered as the storage capacitor 130. A data voltage (e.g., an image signal) is applied to the first electrode 132 and the storage capacitor 130 is charged with the data voltage (e.g., the image signal).
The gate electrode 122 is connected with the first electrode 132 of the storage capacitor 130. Therefore, a potential difference of the first electrode 132 is equal to that of the gate electrode 122.
The output electrode 124 is connected with the second electrode 131 of the storage capacitor 130. The storage capacitor 130 maintains a turn-on state of the driving thin film transistor 120 during an emission interval of the organic light emitting element 160. An anode contact part 161c is connected with the anode 161 of the organic light emitting element 160 (refer to FIG. 1B).
The data line 151 is disposed so as to be adjacent to the storage capacitor 130. The data line 151 is configured to transfer a data voltage (e.g., an image signal). As a gap between lines and the thin film transistor of a high-resolution organic light emitting display device 100 is decreased, a gap between the data line 151 and the first electrode 132 of the storage capacitor 130 may be decreased. As the gap between the data line 151 and the first electrode 132 of the storage capacitor 130 is decreased, a capacitance is formed between the data line 151 and the first electrode 132 of the storage capacitor 130. For convenience in explanation, the capacitance formed between the first electrode 132 of the storage capacitor 130 and the data line 151 is defined as a parasitic capacitance Cp.
To be specific, the data voltage (e.g., the image signal) is supplied to the first electrode 132 through the data line 151 and stored in the first electrode 132 of the storage capacitor 130. The first electrode 132 is in a floating state during an emission interval. During the interval, the data line 151 continuously and sequentially supplies various data voltages to other sub-pixels of the organic light emitting display device in response to a scan signal. The first electrode 132 and the data line 151 are coupled so as to generate the parasitic capacitance Cp. Further, the first electrode 132 is in a floating state and thus affected by the various data voltages supplied to the other sub-pixels.
A potential difference between the first electrode 132 and the data line 151 tends to be maintained by the parasitic capacitance Cp. That is, because the first electrode 132 is in a floating state, the data voltage stored in the first electrode 132 fluctuates due to various data voltages passing through the data line 151 in order to maintain a uniform potential difference with respect to the data line 151 by the parasitic capacitance Cp.
Therefore, a voltage of the gate electrode 122 of the driving thin film transistor 120 fluctuates with the fluctuation of the data voltage stored in the first electrode 132. The voltage of the gate electrode 122 controls the conductivity or electrical resistance of the active layer 121 which is a semiconductor material for regulating an amount of a current. When the voltage of the gate electrode 122 is changed, an amount of a current to be applied to the organic light emitting element 160 is changed accordingly. As a result, the luminance of the organic light emitting element 160 is changed.
In other words, the first electrode 132 of the storage capacitor 130 and the data line 151 are coupled by the parasitic capacitor Cp. That is, due to a change in data voltage applied to the data line 151, a voltage for the first electrode 132 of the storage capacitor 130 may be changed. Particularly, as a difference between the data voltage stored in the first electrode 132 and the current data voltage applied to the data line is increased, a degree of crosstalk increases.
In addition, a capacitor has a characteristic of maintaining a both-end voltage. Thus, if a data voltage applied to the data line 151 is changed, a voltage for the first electrode 132 of the storage capacitor 130 is also changed. Because the first electrode 132 of the storage capacitor 130 is connected with the gate electrode 122 of the driving thin film transistor 120, if the data voltage applied to the data line 151 is changed, a gate voltage of the gate electrode 122 of the driving thin film transistor 120 is also changed. That is, the gate voltage of the gate electrode 122 becomes equal to the data voltage stored in the first electrode 132.
As the gate voltage applied to the first gate electrode 122 of the driving thin film transistor 120 is changed, an amount of the driving current transferred to the anode 161 through the driving thin film transistor 120 is changed. That is, when a potential difference between the gate electrode 122 and the output electrode 124 of the driving thin film transistor 120 are uniformly maintained, the amount of the driving current can be uniformly maintained. However, because the gate electrode 122 of the driving thin film transistor 120 and the data line 151 are coupled with each other, the driving thin film transistor 120 cannot uniformly maintain the driving current. Particularly, as a pixel per inch (ppi) of the organic light emitting display device is increased, such a problem is worsened. Hereinafter, a function of uniformly maintaining an amount of a driving current flowing through the driving thin film transistor 120 will be defined as a current holding ratio (CHR). Hereinafter, a phenomenon that an amount of a driving current flowing through the driving thin film transistor 120 is coupled with an image signal of other sub-pixels and luminance is increased will be defined as crosstalk. As a current holding ratio of the driving thin film transistor 120 is decreased, the luminance of the organic light emitting element 160 is gradually decreased. Further, the luminance of the organic light emitting element 160 is coupled by a data voltage to be applied to the other sub-pixels according to the crosstalk. Thus, an image quality of the organic light emitting display device deteriorates. Further, as a parasitic capacitance is increased, the image quality of the organic light emitting display device deteriorates.